1. Field of the Invention
The invention relates to a case molded capacitor, which is a metallized film capacitor resin molded in a case, and suitably used for a variety of electronic devices, electrical equipment, industrial equipment, automotive vehicles and the like. The capacitor is especially useful for the purpose of smoothing, filter or snubber in a motor driving inverter circuit of a hybrid vehicle.
2. Background Art
In recent years, many kinds of electronic devices are controlled by an inverter circuit in view of the environment protection, especially to save energy and achieve high efficiency. In this circumstance, an automotive industry is particularly introducing to market a hybrid electric vehicle (hereinafter, HEV) which runs with an electric motor with a gasoline engine. As it is typically represented by the HEV case, technique which is benign to earth environment and realizing energy saving and high efficiency is actively pursued.
Working voltage of a HEV motor is several hundreds volts. Because of the high voltage, a metallized film capacitor having a high withstanding voltage and a low loss is getting an attention as a capacitor suitable for the HEV motor. A maintenance-free product is a recent demand, so the metallized film capacitor having an exceedingly long life is remarkably used.
However, when alternating voltage is applied to the metallized film capacitor, a ripple current may be generated and vibrate the capacitor and causing an audible noise. When the metallized film capacitor is used for smoothing in an inverter circuit of HEV, an audible frequency in several to 15 kHz is applied as a switching frequency. So, the noise that the capacitor generates needs to be reduced as much when it is used for a vehicle which requires a high level of quietness.
The noise is entirely different from a beat sound generated by a consumer type metallized film capacitor that is used for a commercial frequency of 50 to 60 Hz. Countermeasure to such commercial type capacitor is thus not effective to the automotive type capacitor, and various proposals are made for reducing the noise generated at high frequency zone.
One way of reducing noise caused by vibration of capacitor is publicly disclosed by Unexamined Japanese Patent Publication No. 2004-303934, in which a production method of a capacitor capable of suppressing a vibration which alternating voltage causes is described. In this method, a capacitor element is manufactured by stacking and winding metallized plastic films, each in which an electrode film is deposited on a dielectric film. The wound capacitor element is then impregnated with liquid insulating material, such as methyl hydrogen silicone oil, a crosslinking reactive monomer of low viscosity of 25 rnm2/s at 25° C. The element is then heated for 15 minutes at 130° C. or 60 minutes at 100° C. so that the methyl hydrogen silicone oil is polymerized to have a high viscosity.
The applicant of this invention disclosed another method of reducing noise of metallized film capacitor in Unexamined Japanese Patent Publication No. 2005-93515. This method reduces noise by putting a metallized film capacitor in a resin case and then resin-molding the capacitor. The method is further explained below.
FIGS. 4A and 4B are a cross-sectional front view and a cross-sectional side view of the metallized film capacitor, showing structure of the capacitor. Sound proofing and sound absorbing material 15 made of foamed urethane is disposed between outside of wound metallized film capacitor element 11 and inside of resin case 12. This structure prevents noise of capacitor element 11 to be propagated outside, enhancing sound insulation capability and reducing noise emission.
According to the conventional metallized film capacitors, the former capacitor controls vibration by itself and reduces noise, and the later capacitor is resin molded in the resin case to reduce vibration and noise. However, the noise is not completely eliminated with either of the method, and further quietness is needed for an HEV application. The metallized film capacitor molded in case 12 in FIG. 4A may be further put in a metal case before it is assembled into a metal inverter box to be installed in a vehicle. In this case, however, the vibration and noise still propagates through a joining part between the metal case and resin case 12 into a vehicle cabin. For reducing the unwanted noise, the vibration needs to be suppressed at much higher level.